The present invention relates to a process for manufacturing a flexographic printing plate.
As is known, a flexographic printing plate comprises a printing form which is made of a material exhibiting a certain amount of compressibility. The resulting structure makes it easier to transfer ink from the printing form to flexible surfaces such as board, paper, polyethylene sheets, etc.
Processes for manufacturing flexographic printing plates involving a laser-etching phase are known. Such processes are described, for example, in EP-A-0 640 043 and EP-A-0 640 044, which disclose that an uncured photopolymer layer, possibly deposited on a flexible support layer, can be covered with an upper layer to constitute a preform. The upper layer, which can be etched using a laser, can be formed, for example, of carbon black. Also disclosed is the use of CO2 lasers and solid-state lasers emitting in the infrared range, such as an Nd:YAG laser, to etch the upper layer of the preform.
The etching phase is then followed by exposure of the photopolymer layer to ultraviolet radiation, making the photopolymer layer accessible through the etched layer. This is then followed by crosslinking of the photopolymer layer, which tends to cure the regions that have been subjected to the ultraviolet radiation. The uncured regions are then removed.
However, this known process has a number of drawbacks resulting from the need to carry out complementary steps of exposure to ultraviolet radiation and of crosslinking. As a result, such a process is costly and lengthy. These steps additionally require the use of chemical solvents, which are the source of consequent pollution.
In accordance with the present invention, a process is provided for manufacturing a flexographic printing plate which serves to alleviate the above-mentioned drawbacks.
To this end, a process is provided for manufacturing a flexographic printing plate in which a layer of a preform is subjected to a laser-etching phase. The layer is made of a material that has been cured before the laser-etching phase, and the laser-etching phase is carried out using a laser that operates at a wavelength of between 200 and 400 nm.
The process of the present invention makes it possible to achieve the above-mentioned objectives by helping to overcome problems resulting from an upper layer attached to the top of the photopolymer material which can be etched by the laser. In accordance with the present invention, the laser-etching face is instead produced directly on the cured material.
In addition, the photopolymer material is subjected to the laser-etching phase after having been cured, rather than before being subjected to curing, as was previously the case. This dispenses with the ultraviolet-exposure and crosslinking phases which were used in the prior process. As a result, when compared with the prior process, the process of the present invention is very advantageous in terms of cost, duration and pollution, especially chemical pollution. The process of the present invention is essentially mechanical, since it does not use the photochemical treatment steps used in the prior process, but rather is carried out by physical machining of the cured material.
Advantageously, the etching phase is carried out at a wavelength of between 248 and 340 nm. The etching phase can be carried out with an excimer laser and with an Nd:YAG laser. Because an Nd:YAG laser usually operates at wavelengths slightly above 1064 nm, it is convenient to bring the frequency of the Nd:YAG laser back into the range which is suitable for implementing the process of the present invention using known techniques which employ harmonic generators to double, triple or quadruple the frequency, correspondingly dividing the emission wavelength by the same factor.
The material which is cured before the etching phase is advantageously made of an EPT (ethylene-propylene terpolymer) material. The EPT material comprises between 55 and 65% by weight of ethylene and between 35 and 45% by weight of propylene, and is filled to between 20 and 40% by weight. The etching phase is carried out with a fluence of between 2 and 4 J/cm2, preferably between 2.5 and 3 J/cm2.
Non-limiting examples of the process of the present invention are further described below, with reference to the following drawings.